Transmission of Electric Signals in Sieve Tubes of Zucchini Plants

Membrane potentials of −;160 to −210 mV were recorded with microelectrodes inserted into meta‐phloem sieve tubes of intact zucchini plants ( Cucurbita pepo L. var. medullosa Alef.). The effects of darkness, white light and colored light on membrane potential were studied. Reference electrodes were in contact with the apoplast via fluid‐filled cavities or “drinks”. Electrolyte solutions (100 mM) in the cavities could be quickly replaced by flushing with 100 mM solutions of sucrose, KCl, sorbitol, or EDTA without altering osmolarity. KCl and EDTA caused depolarization of the sieve tube membrane potential, while sucrose caused depolarization or hyperpolarization of the sieve tube membrane potential in mature or growing plant parts respectively. Recovery of the original voltage was recorded when rapid (sucrose) or slow (sorbitol) transients occurred. When two measuring circuits were installed, one in a growing fruit and the other in the petiole of the subtending mature leaf, the alteration of the sieve tube membrane potential at one site was accompanied by an alteration of the potential at the other site after a few seconds. The responses were opposite in the exporting leaf and importing fruit when sucrose was applied. The signal, transmitted via the sieve tubes, reached maximum velocities of 10 cm per second.